Add the ability to read files from Starlark

[rohansingh]

Starlark scripts can now load arbitrary files from their bundle and read them using a Python-like open() and read() API. A readall() convenience function is also provided.

The examples have been updated to use this functionality where relevant. For example:

load("icon.png", icon = "file")

BTC_ICON = icon.readall()

Supporting changes:

1. The pixlet render command can now accept an entire directory containing multiple Starlark files and static resources.

2. The pixlet private bundle command can now accept an entire directory. It will load the applet contained in the directory, and bundle any files that the applet itself loads. Not that "private" is a misnomer, this command is used by both private and community apps.

3. App manifests no longer contain a fileName attribute, since an app may contain multiple files. packageName has been removed as well, since it is no longer used for anything.

[jmanske]

Holy smokes @rohansingh

When is this going live? Could be game changer for apps that need to render a lot of images.

[rohansingh]

@jmanske Ideally this week. Agreed, it will be great for images and other assets. The primary motivation is to be able to include audio files for Tidbyt Gen 2.

[rohansingh]

Offline review from @matslina:

• Simplify API to only have readall(), since seeking isn't necessary
• Implement serve and bundle.

Deferred for future PR's:

• Have stuff like render.Image could accept a file object, so it’s easy for devs to not do redundant calls to readall. Thinking of apps that load 100 assets, but only show 2 per run depending on config.
• Figure out how the file object will be used in the case of app audio, where we probably don’t want the pixlet app to readall the data on every run. Should files have some internal attribute asset_id set to myapp/icon.mp3 or equivalent, that then gets passed on to firmware?
• Figure out how built-in audio files are referenced. Would user load("tidbyt/skip_forward.mp3", meeting_starts_audio = "file")?